CAUGHT OUT: Catcher John Buck shows his frustration after striking out in the fifth inning of the Mets’ 4-1 loss to the Nationals yesterday.AP

WASHINGTON — The Mets’ rotation officially is a six-pack.

Manager Terry Collins made his plans known before yesterday’s 4-1 loss to the Nationals, adding Jenrry Mejia to a rotation that already includes Matt Harvey, Zack Wheeler, Jeremy Hefner, Dillon Gee and Carlos Torres.

Mejia, who pitched seven shutout innings against the Nationals as a doubleheader call-up on Friday, is scheduled to start against the Marlins on Wednesday, bumping Harvey to Thursday on five days’ rest.

“It’s too early to determine how far we’re going to go out with [a six-man rotation],” Collins said. “It all depends what happens here in the next two or three weeks. If we continue to play pretty good and you look up and we’re five or six [games] out, we’ll have to determine what our next move is.”

The plan calls for Harvey, who has pitched 145 innings this season, to make 10 more starts. Mejia’s addition to the rotation gives Harvey a better shot at finishing the season without skipping starts.

Two weeks ago, the 23-year-old Mejia, who has battled elbow problems over the last three seasons, barely was on the Mets’ radar. But that changed with two strong performances at Double-A Binghamton.

“I was thinking I was going to be here, but not at this time,” Mejia said. “When I was in Binghamton and they told me I was going to be the starter Friday, the only thing I knew was for one start.”

In his seven shutout innings at Nationals Park in Game 1 on Friday, the right-hander allowed seven hits, struck out seven and did not issue a walk. The performance sent another positive vibe through an organization whose future hinges on young, powerful arms.

“With what Jenrry did [Friday], and if he has another outing like that, I think it sends a huge message,” Collins said. “The plan since Sandy [Alderson, general manager] took over is to build up the organization, build up the minor league system and get some of these young, talented, guys up here, and it’s coming to hold true.”

Rafael Montero, Noah Syndergaard, Jacob deGrom and Cory Mazzoni are among the other young arms with highly projected upsides within the organization.

“There’s a list, and they can’t all start,” Collins said. “But when you have quality pitching like that, you’re going to win baseball games.”

Mejia has bone chips in his right elbow, but said the pain is manageable and he expects to finish the season before likely undergoing surgery. He said he hopes to spend the next two months showing the Mets he belongs in their future plans.

Mejia arrived in the big leagues in 2010, pitching mostly from the bullpen before undergoing Tommy John surgery the next year. Mejia made a September cameo with the Mets last year, when he posted a 5.63 ERA in five appearances. Elbow and forearm soreness sidelined him for the first half of this season.

The best sign for the Mets might be the return of Mejia’s cutter, a pitch he said he never has consciously thrown.